1. Field of the Invention
Anti-skid control brake systems and safety devices used therein.
2. Prior Art
A braking system of the similar type described in this application is disclosed in German unexamined patent application DT-OS No. 1,947,012. In the system disclosed therein there is a braking system with an anti-skid control wherein a pressure control device is connected to the connecting lines in a position between the static master cylinder and at least one wheel cylinder. The pressure control device has a wheel cylinder pressure applied piston having a specific surface which is subjected to an auxiliary pressure from a brake fluid under pressure source through a valve which is able to block a connection between the pressure source and the pressure control device as well as to open a return connection. In addition, the device has a valve member which is operable by the wheel cylinder pressure applied piston which is able to block the connection between the master cylinder and the wheel cylinder.
In the device as described in the aforementioned German unexamined application, the piston is designed as a floating piston which in normal circumstances is kept in one of its end positions by means of the auxiliary pressure acting on its bottom side. In that end position the floating piston simultaneously keeps a check valve open which is blocking in the direction of the wheel cylinder. At the on set of a control cycle, the auxiliary pressure acting on the bottom side of the floating piston is reduced by the valve by way of discharging the pressure medium through return lines to a reservoir, by means of which the floating piston is brought into a floating position wherein the check valve is closed. Thus starting from the master cylinder the device prevents any further pressure build up in the wheel cylinders. In this situation the pressure prevailing in the wheel cylinders will only be a determined by the auxiliary pressure acting on the bottom side of the floating piston. Leaving the frictional forces on the floating piston aside, the effective braking pressure will thus behave as if the wheel cylinders were directly communicating with the valve during the control cycle. In the normal situation, that is to say, when there is no control cycle the floating piston is kept in its end position in which position it simultaneously keeps the check valve open. Accordingly, when no control cycle is in operation the auxiliary pressure acting on the floating piston must always exceed the highest braking pressure possible in order to keep the floating piston in its end position. This implies that any control cycle will always start at this high pressure level even if, due to outside circumstances, the limit with regard to wheel lockup is reached at a considerably lower braking pressure. Accordingly the start of the pressure reduction caused by the valve will not come into full operation until some time has passed, with the result that a braking system of that general type is characterized by unduly long idle times especially where low frictional values are involved between the road surface and the wheel contact surface.
Inasmuch as in all such braking systems a normal application of the brakes must be provided for even upon the failure of the auxiliary pressure, the prior art braking system is also provided with an emergency spring. Upon the failure of the auxiliary pressure the spring will act on the floating piston so as to keep it in its end position. If auxiliary pressure is available the emergency spring will be depressed by a clamping piston so as to allow the floating piston to move free of the spring. The design rating of the emergency spring must be very high since, even in the noncompressed state, it must be able to keep the piston in its end position against the highest braking pressure possible which acts on the piston. This means that the assembly in the pressure control device is under considerable tension. Another disdvantage of this prior art braking system consists of the fact that the piston is under permanent action of the auxiliary pressure if the braking system is intact. Thus the seal provided at the piston will also be permanently under pressure. Additionally due to the rarely occurring control operations the piston will remain unmoved for long periods of time. This means that there is a special danger for the seal. If there is damage to the seal the result will be a failure of the braking system at least in the braking circuit which is connected to the respective pressure control device.
A further known disadvantage of the prior art braking system results from the fact that in each case towards the end of any control cycle the control operations will have an effect on the master cylinder and hence can be felt on the brake pedal. This is due to the fact that during the braking operation the brake fluid under pressure available in the wheel cylinder is heated and is thus subjected to an increase in volume. Consequently when the piston is reaching its end position at the termination of any control cycle a small amount of the fluid under pressure is moved back into the master cylinder where it will lead to a pressure peak.